The present invention relates to composite materials having dispersed therein optically transparent solid solution inorganic nanoparticles doped with one or more active ions. The present invention also relates to luminescent devices incorporating the composite materials.
Halide salts have received world-wide attention as materials for a myriad of photonic applications. This results from a chemistry in which the ionic species are of generally greater atomic mass and weaker bonding than oxide-based compounds. This intrinsically results in a greatly enhanced theoretical transparency-hence there is substantial interest from telecommunication companies looking for ultra-low loss halide (predominantly fluoride) optical fibers for long-haul communications. When halide materials are doped with luminescent ions (e.g., the rare-earths), the weak bonding between relatively heavy atoms further results in a reduced influence of the host on the dopant, thereby causing radiative emissions. Accordingly, the halides are said to be of low-phonon energy and thereby enabling of a wealth of applications. Pertinent examples are optical amplifiers at the 1.3 μm telecommunications window, upconversion light sources providing virtually any emission
across the near-ultraviolet, visible, and near-infrared spectrum, color display materials (flat panel phosphors and volumetric monoliths), and long-wavelength sources for infrared imaging, atmospheric sensing, and military counter-measures. Collectively, these few applications represent a multi-trillion-dollar-per-year commerce.
Unfortunately, in most cases, conventional processing methods have failed in their efforts to produce optical components such as fibers with the promised near-intrinsic material properties much less expensively. Resultantly, rare-earth doped halide amplifiers are sold on a very small scale by a very small number of companies. Only applications utilizing relatively small-scale consumption of halide materials currently are sought-generally based on the halides' low-phonon energy nature and resultant luminescent properties.
In particular, conventional processing methods have failed to produce significant concentrations of rare earth element ions in metal halide salts. Jones et al. J. Crystal Growth, 2, 361-368 (1968) discloses that the concentrations of rare earth ions in LaF3 crystals grown from a melt is limited to levels ranging from 25 mole percent for samarium (Sm) to less than 1 mole percent for ytterbium (Yb). Only cerium (Ce), praseodymium (Pr) and neodymium (Nd) are disclosed as being completely soluble in LaF3.
Kudryavtseva et al., Sov. Phys. Crystallogr., 18(4), 531 (1974) disclosed that higher solubilities can be obtained when melt-grown crystals are quenched into water. The disclosed improved solubilities in LaF3 range from 65 mole percent for Sm down to 5 mole percent for lutetium (Lu).
Neither prior art publication discloses the direct preparation of rare earth element doped metal halide salt nanoparticles. A need exists for a method by which such particles may be directly prepared, as well as for materials having increased levels of rare earth element dopants from terbium (Tb) to Lu.
Furthermore, to have a significant optical function where optical transparency is required, light must be able to propagate a reasonable distance with very little of the light attenuated. Optical attenuation is a measure of optical loss and is expressed in units of decibels per unit length. Attenuation is defined on a logarithmic scale, wherein a factor of two difference in attenuation represents a 100-fold difference in intensity.
Prior art phosphor particles are disclosed for use in applications wherein optical transparency is not critical, such as electroluminescent displays, printing inks and biological markers. For purposes of the present invention, an “optically transparent composite material” is defined as a material in which the particles dispersed therein do not scatter wavelengths critical to the end-use application. At the very least, scattering of excitation and emissive wavelength does not occur to the extent that detection of the emissive wavelength for the particular end-use application is impaired. An “optically transparent composite material” may in addition be transparent to visible spectra wavelengths. For example, the transparency required for telecommunications components at conventionally used visible and IR wavelengths differs from that which is considered transparent to the naked human eye. Window glass has an attenuation of 1000 db/km, which is not suitable for telecommunications. The optical fibers used in telecommunications are fabricated from high purity silica glass with an attenuation of 0.2 db/km. Even though both materials are transparent to the naked human eye, only one is suitable for telecommunications.
Optical loss cannot be adequately controlled for telecommunications purposes using prior art phosphor particles disclosed for use in electroluminescent displays, printing inks and biological markers. While particle sizes as small as 100 nm are disclosed, this is the primary size of inorganic particles that agglomerate to form secondary particles significantly greater than 100 nm in diameter. There remains a need for phosphor particles having a dispersed particle size below 100 nm.